The present invention relates generally to a xcex94xcexa3 type A/D converter. More particularly, the invention relates to a xcex94xcexa3 type A/D converter which can switch a measurement range and an input signal.
A xcex94xcexa3 type AD converter permitting switching of a measurement range has been disclosed in Japanese Patent Application Laid-Open No. Heisei 11 (1999)-145829 which discloses an A/D converter performing gain adjustment by varying performance of D/A converting means. Namely, a performance variable D/A converter is provided, in which the performance of the D/A converter is controlled without providing gain variable device on an input side by supplying a control signal to a control terminal of the D/A converter for achieving gain control. On the other hand, the xcex94xcexa3 type AD converter handling a plurality of inputs has been disclosed in Japanese Patent Application Laid-Open No. Heisei 11(1999)-150784, which includes a plurality of microphones converting input acoustic energy into electrical signal in predetermined partial band, a plurality of A/D converting elements converting into digital codes and means for synthesizing outputs of the A/D converting elements.
In the prior art set forth above, no consideration has been made for switching of a measurement range or switching of the input signal. At first, the A/D converter having a gain control function as disclosed in Japanese Patent Application Laid-Open No. Heisei 11-145829 does consider a gain but does not consider an offset. On the other hand, the microphone device consisted of a plurality of partial band microphones as disclosed in Japanese Patent Application Laid-Open No. Heisei 11-150784, it has been required to arrange A/D converters of number corresponding to number of inputs so as to handle a plurality of inputs.
When an input for the xcex94xcexa3 type AD converter is unspecified, it is not possible to see amplitude and offset of the input signal in advance. On the other hand, when a plurality of inputs are used with switching therebetween, amplitudes and offsets of respective inputs may be different. For adapting to this, it can be considered to arrange variable gain amplifier for each input signal at preceding stage of the xcex94xcexa3 type AD converter. However, in this case, scale of the circuit becomes quire large. On the other hand, when A/D conversion is to be performed for a plurality of inputs, if A/D converters in number corresponding to number of inputs are arranged, scale of the circuit becomes unacceptably large. To this fact, the foregoing prior art have given no consideration.
The present invention has been worked out for solving the problems set forth above. It is therefore an object of the present invention to provide a xcex94xcexa3 type AD converter which can arbitrarily set conversion range to permit setting of an optimal conversion range depending upon the input signal.
In order to accomplish the object set forth above, according to the present invention, a xcex94xcexa3 type AD converter comprising:
a local D/A converter having at least a first output voltage level and a second output voltage level;
an integrator integrating a difference of an input signal and an output of the local D/A converter;
a comparator comparing the outputs of the integrator;
means for varying the output of the local D/A converter depending upon an output of the comparator; and
switching means for switching the first output voltage level and the second output voltage level of the local D/A converter.
On the other hand, it may have polarity inverting means for inverting polarity of the output signal of the local D/A converter. Furthermore, according to the present invention, a xcex94xcexa3 type AD converter comprises:
switching means for switching a plurality of input signals
an integrator integrating a difference of an output signal of the switching means and an output of the local D/A converter;
a plurality of capacitors holding an integrated value of the integrator; and
means for switching said capacitors depending upon switching of said switching circuit.